Unidirectional anisotropy - exchange bias

The so-called exchange bias effect is observed for layer systems, which exhibit a mutual in-terface between a ferromagnetic and an antiferromagnetic material. The hysteresis curve of the ferromagnetic layer is shifted with respect to zero applied field. This shift is called ex-change bias field. By means of He ion irradiation it is possible to modify both the magnitude [1] and the direction [2] of this exchange bias field. This effect has been used for both a magnetic patterning [3] and for an improved fabrication scheme for magnetic sensor applications [4].

In the following an example for the magnetic patterning capabilities is shown (from [5]). As a model system the following layer structure is used: glas substrate / 4 nm Ta / 20 nm PtMn / 6 nm NiFe / 4 nm Ta. This layer system is very sensitive to ion irradiation, because PtMn exhibits the desired antiferromagnetism only in its ordered L10 phase. Thus a pure magnetic micro- and nanopatterning is possible with only minimal surface corrugation due to sputtering effects. By means of 25 keV focused Ga+ ion irradiation (IMSA-OrsayPhysics) a stripe pattern (stripe: width = 1 µm, length = 1000 µm, stripe-to-stripe distance = 1µm) with the long axis oriented parallel to the exchange bias direction is fabricated with an ion fluence of 2*1014 Ga/cm2. In addition an area of 300x300 µm2 is homogeneously irradiated with the same fluence.

Figure 1:

Magnetic hysteresis loops at different sample positions: a) after deposition and magnetic field annealing. b) Homogeneously irradiated area and stripe array. The hysteresis loops are off-set for clarity. (from Ref. [5])

In Fig. 1 the hysteresis loops for different sample positions are shown. In the non-irradiated area an exchange bias field of 180 Oe (a); in the homogeneously irradiated area a complete suppression of the exchange bias effect is found (b). Surprisingly the magnetization reversal of the stripe array is not just a simple superposition of the other two hysteresis loops. Due to the exchange interaction between the irradiated and non-irradiated areas the exchange bias field is reduced to about 35 Oe. This demonstrates already that a pure magnetic patterning can lead to modified integral magnetic properties. In order to investigate this behavior in more detail Kerr microscopy (Fig. 2) and magnetic force microscopy (Fig. 3) is used. For certain applied magnetic fields a contrast arising from different magnetization directions is observed. For increasing magnetic field strength the domain walls between the different areas (irradiated and non-irradiated) move in order to maximize the areas with the magnetization direction pointing along the external field direction. The result is a strongly reduced exchange bias field for the stripe array.

Figure 2:

Kerr microscopy images of the stripe array. At the beginning the magnetization direction is homogenously pointing upwards. At a magnetic field strength of -13.6 Oe a strong contrast is observed. The magnetization directions in the irradiated areas are now aligned antiparallel to the surrounding non-irradiated stripes. Upon further increase of the magnetic field the areas with a magnetization direction parallel to the applied field grow at the expense of the intervening areas with antiparallel magnetization direction. The width of the dark stripes shrinks. The continuous domain wall motion is also consistent with the gradual reduction of the magnetization in Fig. 2 b). For an applied field larger that -102 Oe the magnetization directions in the non-irradiated areas are also aligned with the applied magnetic field. The hysteretic behavior is observed due to the appearance of maximum contrast for different applied fields on both branches of the loop. (from Ref. [5])